https://protectingchildren.google/intl/en/

> CSAI Match is our proprietary technology, developed by the YouTube team, for combating child sexual abuse imagery (CSAI) in video content online. It was the first technology to use hash-matching to identify known violative videos and allows us to identify this type of violative content amid a high volume of non-violative video content. When a match of violative content is found, it is then flagged to partners to responsibly report in accordance to local laws and regulations. Through YouTube, we make CSAI Match available for free to NGOs and industry partners like Adobe, Reddit, and Tumblr, who use it to counter the spread of online child exploitation videos on their platforms as well.

> We devote significant resources—technology, people, and time—to detecting, deterring, removing, and reporting child sexual exploitation content and behavior. Since 2008, we’ve used “hashing” technology, which creates a unique digital ID for each known child sexual abuse image, to identify copies of images on our services that may exist elsewhere.